1. Field of the Invention
The present invention relates to a technique of adjusting light power incident on an optical transmission line, and more particularly to a light power adjusting method, an optical transmitter, and an optical receiver suitable for ensuring high data transmission quality by preventing a rapid increase in SBS (Stimulated Brillouin Scattering) which occurs when the light power exceeds a predetermined value.
2. Description of the Related Art
An SMF (Single Mode Fiber), which is one of optical fibers forming an optical transmission line, is divided into such types as SSMF (Standard SMF), DSF (Dispersion Shifted Fiber), NZ-DSF (Nonzero DSF), and so forth, depending on differences in a chromatic dispersion coefficient and an effective cross section area Aeff of a core.
The optical fiber type is closely related to the power of the light incident on the optical fiber. Therefore, even if a certain incident light power does not cause any problem to one type of optical fiber, the incident light power may cause a rapid increase in a nonlinear phenomenon SBS to another type of optical fiber, and thus may cause serious deterioration of the data transmission quality. A value of the incident light power at which SBS rapidly increases is referred to as a light power threshold of the rapid SBS increase.
FIG. 1 illustrates an overview of occurrence of SBS.
SBS occurs as a result of interaction between light and acoustic waves (i.e., phonon) occurring in glass forming an optical fiber. Further, SBS is a nonlinear scattering phenomenon, and the occurrence rate of SBS rapidly increases when the power of light incident on the optical fiber exceeds a predetermined value. As illustrated in FIG. 1, when SBS occurs, SBS-induced backward propagating light 23 is generated in a direction opposite to a direction of incident light 10 output by an optical transmitter 100 to an optical transmission line 300a. Thus, the incident light 10 incident on the optical transmission line 300a reaches an optical receiver 200, with a part of the incident light 10 lost as the SBS-induced backward propagating light 23. Therefore, when the SBS-induced backward propagating light 23 rapidly increases, the rate of errors in data reception by the optical receiver 200 increases. As a result, the data transmission quality is deteriorated.
The occurrence rate of SBS varies depending on the Aeff value of the optical fiber and a power level of the light incident on the optical fiber. The occurrence of the SBS phenomenon can be suppressed by reducing the power of the incident light 10 or by applying a low-frequency amplitude modulation to the incident light 10 and thus decreasing energy density per wavelength. Further, the light power threshold of the rapid SBS increase varies depending on the Aeff value of the optical fiber, which depends on the optical fiber type. That is, if the optical fiber type is previously known, and if the incident light 10 is set to have an optimal light power smaller than the light power threshold of the rapid SBS increase for the optical fiber type, the increase in the SBS reflected light (i.e., the SBS-induced backward propagating light 23) can be suppressed. Accordingly, high data transmission quality can be ensured.
In a known technique, an operator of an optical transmission system is inquired to as to the optical fiber type he uses, and the output level of the optical transmitter is set on the basis of the information of the optical fiber type. This technique, however, involves extra trouble to confirm with the operator, and the information may not always be obtained.
In another known technique, the optical fiber type is estimated by measuring the power of the SBS-induced backward propagating light 23 returned from the optical transmission line 300a when the optical transmitter 100 is started. Then, the output power of the light output by the optical transmitter 100 to the optical transmission line 300a is automatically set.
FIG. 2 illustrates an example configuration in which reflected light 20 is measured by the optical transmitter 100. The reflected light 20 returned from the optical transmission line 300a is received by a reflected light measuring unit 130 via an optical coupler 101, and then the light power of the reflected light 20 is measured.
In this example, the optical fiber type is determined on the basis of the absolute value of the weak light power of the reflected light 20 returned from the optical transmission line 300a. However, the reflected light 20 returned from the optical fiber includes Fresnel reflected light 22, Rayleigh scattering light 21, and the like, in addition to the SBS-induced backward propagating light 23. Due to influences of such light, therefore, it is difficult to obtain an accurate light power value of the SBS-induced backward propagating light 23. As a result, it is highly possible to erroneously recognize the optical fiber type.
That is, the reflected light 20 returned from the optical transmission line 300a mainly includes the Rayleigh scattering light 21, the Fresnel reflected light 22, and the SBS-induced backward propagating light 23. Thus, the measured power of the reflected light 20 is equal to the absolute value of the sum of the light powers of the Rayleigh scattering light 21, the Fresnel reflected light 22, and the SBS-induced backward propagating light 23. Therefore, the light power value of only the SBS-induced backward propagating light 23 cannot be accurately calculated. Thus, it is also difficult to accurately identify the type of the optical fiber forming the optical transmission line 300a. If the optical fiber type is thus erroneously identified, and if the power of the output light output by the optical transmitter 100 is inappropriately set (i.e., set at a value larger than the threshold of the rapid SBS increase), the inappropriate light power may cause the rapid increase in SBS in the optical transmission line 300a. As a result, the data transmission quality may be seriously deteriorated.
As a technique of detecting and preventing the rapid increase in SBS, Japanese Unexamined Patent Application Publication No. 9-33389 (pages 3-4 and FIG. 7) describes a technique of detecting deterioration of an optical fiber by detecting scattering light caused by SBS. Further, Japanese Unexamined Patent Application Publication No. 10-200483 (pages 3-4 and FIGS. 1 and 3) describes a technique of preventing the rapid increase in SBS by adjusting the amount of a frequency modulation applied to a light signal.